AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Astrophysical Techniques IV –Spectroscopy
B. Nikolichttp://www.mrao.cam.ac.uk/˜bn204/mailto:[email protected]
Astrophysics Group, Cavendish Laboratory, University of Cambridge
February 2012
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Outline
Science background
Spectroscopy techniques
Back matter
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Science
I Spectroscopy is a key tool in many areas ofastronomy from astrophysics
I It is used from the shortest to longest wavelengthsobserved in astronomy
I A wide range of instruments is available to matchpossible science goals
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
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Science areas
I Measurement of redshift (of distant galaxies, quasars,etc)
I Quantitative measurement of recent star formation innear-by galaxies
I Measurement of metallicity of starsI Composition, chemistry, physical conditions of the
interstellar mediumI Dynamics of galaxies, clusters of galaxies, the
interstellar mediumI Radial velocity planet searchesI Studies of interstellar dust through extinction
measurementsI Many more! Most imaging studies lead to
spectroscopic followup
AstrophysicalTechniques IV
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‘Damped’ Lyα systems
Sloan Digital Sky Survey spectrum of a Q1135-0010
4000 5000 6000 7000 8000 9000Observed Wavelength
-5
0
5
10
15
20
25
Flu
x
(1e-
17
cg
s u
nit
s)
DLA
From:[Kulkarni et al.(2012)Kulkarni, Meiring, Som, Peroux, York, Khare, and Lauroesch]
AstrophysicalTechniques IV
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Spectroscopytechniques
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‘Damped’ Lyα systems
VLT/UVES zoom in
3700 3800 3900 4000 4100
0
0.5
1
1.5
Ob
serv
ed F
lux
(1
e-1
6 c
gs
un
its)
3700 3800 3900 4000 4100Observed Wavelength
0
0.5
1
1.5
No
rmal
ized
Flu
x
log NHI = 22.05
log NHI = 21.95, 22.15
log NHI = 21.85, 22.25
From:[Kulkarni et al.(2012)Kulkarni, Meiring, Som, Peroux, York, Khare, and Lauroesch]
AstrophysicalTechniques IV
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Spectroscopytechniques
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Gas outflows from proto-stars
JCMT/HARP-B CO 3→2(Focal Plane heterodyne array)
0
2
4
6
8
10
-50 -40 -30 -20 -10 0 10 20 30 40
12C
O(3
-2)
Tb [K
]
vLSR [km s-1
]
Blueshifted LobeRedshifted Lobe
From:[Gottschalk et al.(2012)Gottschalk, Kothes, Matthews, Landecker, and Dent]
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
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Stellar dynamics of nearby galaxies
LBT ‘long-slit’ near-IR spectroscopy
From: [Greco et al.(2012)Greco, Martini, and Thompson]
AstrophysicalTechniques IV
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Sciencebackground
Spectroscopytechniques
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Stellar dynamics of nearby galaxies
LBT ‘long-slit’ near-IR spectroscopy
From: [Greco et al.(2012)Greco, Martini, and Thompson]
AstrophysicalTechniques IV
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Spectroscopytechniques
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Tracing star formation and dynamics usingHα
VLT/VIMOS IFU spectroscopy
From[Bremer et al.(2012)Bremer, Scharwachter, Eckart, Zuther, Fischer, and Valencia-S]
AstrophysicalTechniques IV
B. Nikolic
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Spectroscopytechniques
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Tracing star formation and dynamics usingHα
Fabry-Perot imaging
From[Dicaire et al.(2008)Dicaire, Carignan, Amram, Marcelin, Hlavacek-Larrondo, de Denus-Baillargeon, Daigle, and Hernandez]
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Outline
Science background
Spectroscopy techniques
Back matter
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Things to think about
1. Spectral resolution2. Range of wavelengths that needs to be covered3. Size/location/distribution of objects4. Spatial resolution5. Dynamic range6. Sensitivity
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
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Basic approaches
1. Interfere incoming radiation with itself withdifferent/variable path length(Optical/infrared/sometimes in sub-mm). Dispersiontechnique:
I Refractive index (Prisms/Grisms)I Diffraction (Gratings)I Variable path length (Fourier transform
spectroscopy/Fabry-Perot)
2. Voltage measurement (possibly after down-mixing)and do spectral analysis in electronics(mm-wave/radio)
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Spectral resolution
Resolving power ≡ R =λ
δλ=
ν
δν(1)
R ∼ ∆Lλ
(2)
where ∆L maximum path length inside the instrument
Type R
Interference filter 102 – 103
Diffraction grating 103 – 106
Fabry-Perot 104 – 106
Fourier Transform 104 – 106
AstrophysicalTechniques IV
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Interference filter
I Essentially a fixed version of Fabry PerotI Layers created by thin film deposition onto glassI Astronomical workhorse
AstrophysicalTechniques IV
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Spectroscopytechniques
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Diffraction Grating
d
Constructive interference when:
d sin(θ) + d sin(θ′) = mλ (3)
d Spacing between rulingsθ Incident angle
m Diffraction “order”λ Wavelength
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Important mathematical relations
R =λ
δλ= Nm (4)
whereN: Number of rulingsm: Diffraction order used
If not diffraction limited then:
R =λ
δλ=
Nmχλ/D
(5)
whereχ: Seeing/angular size of slit usedD: Telescope diameter
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Some practicalities
I Artifacts from bright sources are common(reflections/ghosting/etc)
I Accurate positioning of slit/fibre/mask is crucial andcan be challenging
I Wavelength calibration is usually required in additionto usual detector calibrations
I Dispersion of incoming radiation over many pixelsmeans that measurements take much longer to be‘background limited’
I Outcome from newer instruments are large data‘cubes’ – can be difficult to interpret and analyse whenthere are many lines/features
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
Outline
Science background
Spectroscopy techniques
Back matter
AstrophysicalTechniques IV
B. Nikolic
Sciencebackground
Spectroscopytechniques
Back matter
References
M. Bremer, J. Scharwachter, A. Eckart, J. Zuther,S. Fischer, and M. Valencia-S.Optical Integral Field Spectroscopy of NGC 5850.ArXiv e-prints, Feb. 2012.
I. Dicaire, C. Carignan, P. Amram, M. Marcelin,J. Hlavacek-Larrondo, M.-M. de Denus-Baillargeon,O. Daigle, and O. Hernandez.Deep Fabry-Perot Hα Observations of NGC 7793: AVery Extended Hα Disk and A Truly Declining RotationCurve.AJ, 135:2038–2047, June 2008.doi: 10.1088/0004-6256/135/6/2038.
M. Gottschalk, R. Kothes, H. E. Matthews, T. L.Landecker, and W. R. F. Dent.The JCMT 12CO(3-2) Survey of the Cygnus X Region:I. A Pathfinder.ArXiv e-prints, Feb. 2012.
J. P. Greco, P. Martini, and T. A. Thompson.Measurement of the Mass and Stellar PopulationDistribution in M82 with the LBT.ArXiv e-prints, Feb. 2012.
V. P. Kulkarni, J. Meiring, D. Som, C. Peroux, D. G.York, P. Khare, and J. T. Lauroesch.A Super-Damped Lyman-alpha QSO Absorber atz=2.2.ArXiv e-prints, Feb. 2012.